The present invention generally relates to a method of image acquisition and lighting techniques for machine vision applications. More specifically, the invention relates to an image acquisition system and method for acquiring multiple images of an object.
Lighting is crucial to the success of machine vision application. A large number of lighting or illumination methods exist that are applicable to machine vision. Some of the commonly used lighting methods can be classified into three categories: back lighting, front lighting, and structured and filtered lighting.
Back lighting is light that comes from the opposite side of the object from which the video source views. Back light which is bright produces a silhouette of an opaque object. The surface of the object is not lit up. For the case of transparent objects, such as plastics and glass, back light is capable of revealing inclusions and scratches. Back lighting provides the highest contrast resulting in a binary image.
Front lighting is lighting that comes from the same side of the object from which the video source views. Front lighting is capable of revealing surface features with different shades of gray or color. Front lighting can be further divided into various configurations, such as directional front light, uniform front light, or onaxis or episcopic front light.
Structured and filtered lighting is a mode of lighting that is usually applied in the front lighting way with filters or patterned gratings attached to the light path. For example, a single or multiple sheet of light can be generated and be projected onto the object surface. One application of such a lighting method is to compute the surface height of the object. This is known as shape from structured light in the art of computer vision. Structured light can also be generated from a laser source with appropriate optics. On the other hand, a polarizer and/or color filters can be used to generate polarized and narrow waveband light.
Most of the time, multiple lighting arrangements are required in an application. For example, back lighting as well as front lighting may be required in a single application. The role of the back light is to highlight the silhouette or contour of the object whereas front light can reveal the surface reflectance and conditions of the object. These two types of lighting cannot exist together as they will cancel the effect of one another. Due to the fact that multiple lighting arrangements cannot be simultaneously applied to the scene, each of the lighting arrangements has to be applied sequentially. Such a sequential mode of lighting application may not be feasible due to time constraints and also due to resultant increasing system complexity and cost.
Known attempts have resorted to multiple stations with separate lighting, cameras and optics when multiple lighting techniques are required to inspect the same object. This arrangement is costly in initial set-up as well as maintenance. Switching of light sources has also been attempted. For example, in the case of both back lighting and front lighting, the first image can be taken with the back light source on while the front light source is off. The second image is taken with the front light on and the back light off. This method requires additional dimension of control for the light sources and may incur reliability problems as lights are being turned on and off rapidly.
A need, therefore, exists for an improved image acquisition system and method for machine visions applications.
It is, therefore, an object of the present invention to ameliorate this problem and to provide an image acquisition system which, in a preferred form, is able to acquire a plurality of images of an object simultaneously with each image relating to a different lighting technique. A further object of the present invention is to provide a method for acquiring such images.
In an embodiment, the present invention provides an image acquisition system having viewing means, separating means operable to separate light incoming from the viewing means into a plurality of light paths, and an image sensor located in each of the light paths. The system is configured such that each of the image sensors is operable to acquire an image from a predetermined wavelength band of the light in its respective light path.
In another embodiment, the present invention provides a method of acquiring a plurality of images of an object. The method comprises the steps of: lighting the object using a plurality of light sources each being limited to a predetermined wavelength band; viewing the object through viewing means; separating the incoming light from the viewing means into a plurality of light paths; and acquiring an image of the object from a predetermined wavelength band of the light in each of the light paths.
In a preferred embodiment of the present invention, different lighting methods are differentiated by light sources of different bands of wavelength. These different bands of wavelength produced by the light sources have minimum or no overlap. These different lighting techniques are applied simultaneously to illuminate the object.
It is further provided that the image acquisition system viewing the object is capable of separating the different bands of wavelength of light either transmitted or reflected from the object into different paths with different image sensors. The system is arranged such that each of the image sensors captures the image of the object illuminated by different lighting techniques. Different image analysis techniques are then applied to the images acquired by the image sensors on the different image paths. A preferred form of the invention hence provides a means to view the object illuminated by multiple lighting techniques from a single position.
In an embodiment, the image acquisition system includes an objective lens as the viewing means and a beam splitter that splits the incoming light into multiple paths. The objective lens provides a common aperture and field of view for the multiple image paths. Each of the multiple paths are wavelength limited preferably by filters and have minimum overlap response with respect to one another. Image sensors are used to capture the images from each of the image paths. It is further provided that optional filters, such as a neutral density filter, can be inserted in the different image paths to control the intensity. A frame grabber may be used to digitize the images captured by the image sensors and send them to the processor for image processing and analysis.
These and other advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings.